Because you become transgressors of whole law, not just of one commandment. Crossing the line or boundary of the law is done by failing on any command. There is no such thing as a limit of commands to be broken, in order to become transgressors.
The Message version puts it like this:
You can’t pick and choose in these things, specializing in keeping one or two things in God’s law and ignoring others. The same God who said, “Don’t commit adultery,” also said, “Don’t murder.” If you don’t commit adultery but go ahead and murder, do you think your non-adultery will cancel out your murder? No, you’re a murderer, period.
James 2:10: “For the person who keeps all of the laws except one is as guilty as a person who has broken all of God’s laws.”
We can understand the sense of it as “whoever keeps the whole law but fails in one point it is as though he has become guilty for all of it.” Your keeping of 99% of the law is not gonna save you. Your obedience to "most" commands is worthless. The point is about slackness of the law, lawlessness or libertinism or licentiousness.
Matthew 5:29: “If your right eye causes you to sin, tear it out and throw it away. For it is better that you lose one of your members than that your whole body be thrown into hell.”
Deuteronomy 27:9-10,17,26; 28:1: “Then Moses and the Levitical priests said to all Israel, “Keep silence and hear, O Israel: this day you have become the people of the LORD your God. You shall therefore obey the voice of the LORD your God, keeping his commandments and his statutes, which I command you today.” … “‘Cursed be anyone who moves his neighbor’s landmark.’ And all the people shall say, ‘Amen.’ … “‘Cursed be anyone who does not confirm the words of this law by doing them.’ And all the people shall say, ‘Amen.’ “And if you faithfully obey the voice of the LORD your God, being careful to do all his commandments that I command you today, the LORD your God will set you high above all the nations of the earth.”
Deuteronomy 28:15: ““But if you will not obey the voice of the LORD your God or be careful to do all his commandments and his statutes that I command you today, then all these curses shall come upon you and overtake you.”
Think about it like this. The penalty of one murder is death, and the penalty of 100 murders is also death. You become guilty of death by committing just one.
John described the definition of sin in his epistles in great details, all wrongdoings are sin but not all sin leads to death. Bridgeway Bible dictionary defines Sin as:
Sin The Bible refers to sin by a variety of Hebrew and Greek words. This is partly because sin may appear in many forms, from deliberate wrongdoing and moral evil to accidental failure through weakness, laziness or ignorance (Exo 32:30; Pro 28:13; Mat 5:22,28; Rom 1:1-32; Jam 4:17). but the common characteristic of all sin is that it is against God (Psa 51:4; Rom 8:7). It is the breaking of God’s law, that law being the expression of the perfection that God’s absolute holiness demands (Isa 1:2; 1Jo 3:4). It is the ‘missing of the mark’, that ‘mark’ being the perfect standard of the divine will (Deu 9:18; Rom 3:23). It is unbelief, for it rejects the truth God has revealed (Deu 9:23; Psa 1:78-22; Joh 1:3-19; Joh 8:24; Joh 16:9). It is ungodliness, and it makes a person guilty before God (Psa 1:1-6; Rom 1:18; Jam 2:10).